While nonwoven glass fiber air filtration products have been made with bi-modal fiber diameter distributions as disclosed in U.S. Pat. No. 5,454,848, issued Oct. 3, 1995, entitled “METHOD OF MAKING AIR FILTRATION MEDIA BY INTER-MIXING COARSE AND FINE GLASS FIBERS” and U.S. Pat. No. 6,227,009 B1, issued May 8, 2001, and entitled “METHOD OF MAKING LONG, FINE DIAMETER GLASS FIBERS AND PRODUCTS MADE WITH SUCH GLASS FIBERS”, the subject invention relates to a nonwoven fibrous insulation product with at least a bi-modal fiber diameter distribution that can provide the fibrous insulation product with a unique and enhanced combination of thermal and other desirable properties.